1. Field of the Invention
The present invention relates to a method for operating a ballast for gas discharge lamps, wherein the ballast comprises a line-voltage rectifier generating a rectified voltage, an inverter fed by the rectified voltage and generating an AC voltage with an inverter frequency, in combination with a lamp in a lamp circuit driven by the AC voltage.
2. Discussion of the Background of the Invention and Material Information
Ballasts of the previously-noted type, for gas discharge lamps, feed a rectified line voltage to an inverter, which converts the rectified voltage back to an AC voltage. This AC voltage is used to operate one or more lamps in a resonant lamp circuit. The frequency of the AC voltage, i.e. the inverter frequency, usually lies close to the resonance frequency of the lamp circuit.
It has been determined that the lamp power of such devices depends strongly on the value of the rectified voltage. Therefore, it has previously been important to smooth the rectified voltage in order to reduce the residual ripple. This in turn requires the use of expensive components, usually comprising large coils and capacitors.
Even if the rectified voltage is smoothed carefully, lamp power still depends on the constancy of the effective line voltage.
Prior art European Patent Publications EP 178 852, EP 239 420 and EP 338 109 describe ballasts that regulate the inverter frequency to keep the lamp current constant or the lamp power constant. This allows regulation of the light flux generated by the lamp. The corresponding regulators are, however, comparatively expensive.